The present invention relates to medical electrode leads and methods of manufacture thereof. The invention has particular utility in connection with cardiac pacing and defibrillation leads, i.e. suitable for intercardial stimulation of the heart with the help of an implantable pacemaker or defibrillator, and will be described in connection with such utility, although other utilities are contemplated.
Surgically implanted cardiac devices play an important role in the treatment of heart disease. In the 50 years since the first pacemaker was implanted, technology has improved dramatically, and these devices have saved or improved the quality of countless lives. Pacemakers treat slow heart rhythms by increasing the heart rate or by coordinating the heart's contraction for some heart failure patients. Implantable cardioverter defibrillators stop dangerous rapid heart rhythms by delivering an electric shock. As the range of applications widens, the number of patients with cardiac devices continues to increase. Approximately 400,000 devices are implanted each year in the United States, and there are >3 million patients with implanted cardiac devices currently living in the United States.
Surgically implanted cardiac devices comprise two main parts, the pulse generator, a metal package that contains electric circuits and a battery, which usually is placed under the skin or on the chest beneath the collarbone, and the wires, or leads, which run between the pulse generator and the heart. In a pacemaker, these leads allow the device to control the heart rate by delivering small busts of electric energy. In a defibrillator, the leads allow the device to deliver a high-energy shock and convert dangerous rapid rhythms (ventricular tachycardia or fibrillation) back to a normal rhythm.
Although leads are designed to be implanted permanently in the body, occasionally leads fail due, for example, to a break in the insulation. In fact, in the last several years, two major manufacturers, Medtronic and St. Jude have recalled cardiac leads due to insulation failure or short circuit.
Cardiac leads typically are formed of concentrically stranded small via wires 2 formed from biocompatible, corrosion resistant, conductive materials such as MP 35 N, a cobalt based alloy having a nominal composition of 35% Ni, 35% Co, 20% Cr and 10% Mo around a conductive biocompatible, corrosion-resistant core 4 formed of, e.g. silver (FIG. 1). A typical lead is constructed with seven strands of MP35 N at 0.005 inches (127 microns) in diameter, cabled, and then formed into a helical spring shape hollow cable or coil as shown in FIG. 2. Other cobalt-chromium alloys also have been used. This cable assembly is then electrically insulated with a suitable dielectric material 6 such as polyethylene or silicone rubber. The insulation also must be biocompatible and corrosion resistant. Most important, the insulation must be flexible and abrasion resistant in order to survive the tens of millions of flexure cycles the leads would be exposed to over the lifetime of a patient.
Failure of prior art cardiac leads typically is as a result of failure of the insulation due to abrasion by the coil. That is to say, a major factor contributing to the insulation failure is due to the design of the hollow lead cable. As can be seen in FIG. 2, the outer strands in contact with the insulation are extensively corrugated and oriented transverse to the axis of the cable. So rather than sliding, the insulation is abraded, by the multiple exposed strands of the hard MP 35N metal wires and excessive wear can occur at tight bends and turns. Thus, while the conductive wires or fibers typically do not fracture, through continuous flexture and bending especially at tight bends and turns, the conductive wires or sheath material would sometimes wear and break through the insulation. As reported in the New York Times, Business Day Section, Wednesday, Aug. 22, 2012, Page B1; the lead failure was due to “inside out” abrasion where the wires had pushed through from the inside. Prior attempts to address the break-through problem, i.e., in terms of improving insulation composition and insulation thickness, have not proved to be entirely satisfactory. Making the insulation thicker will make the lead more robust; however, making the insulation thicker compromises flexibility which may present problems to the surgeon during implantation. Also, the leads must be sufficiently flexible once implanted so as to not exert stress on or injure body parts surrounding the implanted electrode.